Tractor loaders generally include one or a pair of lift arms pivotable at one end on the tractor or tractor framework and pivotally supporting the bucket at the other end. The lift arm or arms can be raised by means of a hydraulic ram or rams acting between the tractor and the lift arms. The bucket can be pivoted fowardly or rearwardly by one or more bucket rams acting through a parallelogram linkage.
The normal operation of a loader involves positioning the bucket to a "dig" or working position and then forcing the bucket into a pile of material by the forward motion of the tractor. The bucket is then pivoted or "rolled back" on the lift arm while the lift arm is raised to force or break out the mass of material within the bucket from the remainder of the pile. Subsequently, the lift arms are elevated a sufficient distance to raise the bucket above the ground and then the vehicle is driven to some other location. Upon reaching the subsequent location, the bucket is pivoted to a "dumping position" where the contents are discharged after which the operation is repeated.
Unless the loader linkage employs a so-called "self-leveling feature," as the bucket is raised, the bucket itself tends to rotate about the lift arms in the same direction as the lift arms are rotating about the frame of the tractor. In other words, with an ordinary tractor loader linkage, the tendency for the bucket is to spill its contents as it is raised above the tractor. To prevent the spillage of the material contained in the bucket, the loader operator has to rotate the bucket relative to the lift arms in the opposite direction that the lift arms are tending to rotate the bucket.
Several linkages have been devised that automatically level the bucket as the lift arms are raise. One common loader linkage having a self leveling function utilizing a "cross-over lever"--a link pivoted intermediate its ends--on each lift arm. The shovel loader described by E. A. Drott in U.S. Pat. No. 2,482,612 is one example. Typically, one end of the cross-over lever is pivotally linked to the bucket and the other end is pivoted to a hydraulic actuator or bucket ram which is itself pivotally connected to the frame of the loader.
A careful study of the loader linkage will show that the "self-levelling" of the bucket is only approximate. In other words, there are certain positions in the raising and lowering cycle of the lift arms during which the counter rotation applied to the bucket by the cross-over lever is more than what is necessary to keep the bucket essentially level; there are also positions where the amount of counter rotation is less than what is necessary to maintain the bucket essentially level.
Another disadvantage of a self-leveling loader linkage incorporating only a cross-over lever is that various parts of the linkage tend to foul each other unless the lift arm and the linkage tilting the bucket are transversely offset from each other. Although offsetting the linkage on either side of the lift arm tends to solve the fouling problem, it creates still another problem. Such an off-set imparts "load couples" to the linkage which are difficult to counteract. These load couples tend to cause undue stress and wear on the linkage and especially the pivot pins. Moreover, offsetting the bucket ram often interferes with the driver's view of the bucket and further complicates the design of the loader linkage.
Turning to the hydraulic actuator or bucket ram that actually tilts the bucket, the bucket ram performs two functions: (1) It rolls back the bucket to fill the bucket when the bucket and the lift arms are in the lowered position; and (2) It tips or tilts the raised bucket forward to dump the contents of the bucket. Comparatively speaking, the greatest load imposed upon the bucket ram is when rolling back a heavily loaded bucket. Very little force is required to tip the raised bucket since, for the most part, gravity is assisting the bucket ram. Consequently, the loader linkage should be arranged in such a manner that the maximum available force from the bucket ram can be used to roll back the bucket. In addition, to avoid load couples and unbalanced moments tending to distort or otherwise bind the loader linkage, the longitudinal axis or the line of force produced by the bucket ram should be parallel to and in the same vertical plane as the link tilting the bucket about the lift arm.
Thus, it should be apparent that there are two problems associated with the design of a loader linkage: (1) Preferably, the linkage should be self-leveling over the entire range of the raising and lowering cycle of the lift arms; and (2) The loader linkage should provide the maximum force available to roll back the bucket during loading. A loader design providing an optimum solution to these two problems would increase the efficiency of the loader and the productivity of the machine's operator.